His lifetime sales are creeping towards the £15million mark, and over the past 12 months, his children's books have sold 1.1 million copies, putting him behind only Julia Donaldson (2.7m) and Jeff Kinney (1.6m), both of whom have far more books on the market.

It means he has outstripped both the perennially successful Jacqueline Wilson and Suzanne Collins, whose Hunger Games trilogy has been a chart-topper for the last few years. Without doubt, he's up there in the Rowling league. How has he achieved such a commanding position, and built on it, year on year, in such a fickle and tempestuous market?

Walliams naturally had an enormous advantage as a debut children's author - he started out from a position of unusual power as a well-known and highly bankable comedy star. But comedic fame doesn't automatically translate to the household name status he's since achieved with his kids' writing. Compare Ricky Gervais's Flanimals - though it was initially a strong seller, a proposed ITV Claymation series of the animal miscellany has now been cancelled, and the big-screen version is mired in development hell . What has Walliams done so differently?

Many reviewers and readers have compared his work to Roald Dahl's - more readily because his first two books, The Boy in the Dress and Mr Stink, are illustrated by Dahl's long-time collaborator Quentin Blake. Walliams acknowledges having saturated himself in Dahl's "perfect" work, and the debt is definitely discernible. Mr Stink's egg-and-sausage-festooned beard, for instance, is straight out of The Twits, and casts of obnoxious siblings, tiresome grandmothers and child-hating head-teachers will ring a whole tower of bells for any Dahl aficionado.

But there's more to Walliams's work - and to his extraordinary success - than simply being an apt pupil of a past master. What he does right in his books is to balance pungent and frequently regrettable comedy, of the sort that particularly appeals to younger male readers, with profound, genuine heart - tenderer feelings, deeper unspoken loves than usually appear in Dahl's more dazzlingly imaginative, savage universe. In The Boy in the Dress, understatedly tragic moments drift by: Dennis's dream of becoming a pro footballer "quietly floating away" now that there's no-one to ferry him to practice, eating "bowls of cereal, even when it wasn't breakfast" since Mum - in charge of nutrition - upped sticks. Walliams's assurance and humanity come across in these quiet lines, even while he retains a keen grasp of such essentials as aggressive burping (the moment when Mr Stink subdues bullying Rosamund, who's just blown £500 of daddy's dosh in Topshop, with a putrid belch whose echoes last a page and a half, made me involuntarily squeal with delight).

As Walliams points out, too, he lacks Dahl's absolute confidence in world-creation - he uses specific references to brands and products to help with settings' authenticity, which open his books up to the risk of premature dating and falling by the wayside unread. Many of his chosen goods, however, are already slightly out of date - Um Bongo, the Teenage Mutant Ninja Turtles - creating a faintly elegiac, wistful sense that the books, for all their contemporary preoccupations, are set somewhere in the author's own childhood.

Writers are often enjoined to "write for the child they were" in order to win over young readers; Walliams, blithely celebrating courage, kindness, tolerance and insanely noxious bodily effluvia, may well have done just that - as young reviewers on the Guardian children's books website will attest.